Measuring Local Electric Fields and Local Charge Densities at Electrode Surfaces Using Graphene-Enhanced Raman Spectroscopy (GERS)-Based Stark-Shifts

Utilization of Electric Fields to Modulate Molecular Activities on the Nanoscale: From Physical Properties to Chemical Reactions

As a primary energy source, electricity drives broad fields from everyday electronic circuits to industrial chemical catalysis. From a chemistry viewpoint, studying electric field effects on chemical reactivity is highly important for revealing the intrinsic mechanisms of molecular behaviors and mastering chemical reactions. Recently, manipulating the molecular activity using electric fields has emerged as a new research field. In addition, because integration of molecules into electronic devices has the natural complementary metal-oxide-semiconductor compatibility, electric field-driven molecular devices meet the requirements for both electronic device miniaturization and precise regulation of chemical reactions. This Review provides a timely and comprehensive overview of recent state-of-the-art advances, including theoretical models and prototype devices for electric field-based manipulation of molecular activities. First, we summarize the main approaches to providing electric fields for molecules. Then, we introduce several methods to measure their strengths in different systems quantitatively. Subsequently, we provide detailed discussions of electric field-regulated photophysics, electron transport, molecular movements, and chemical reactions. This review intends to provide a technical manual for precise molecular control in devices via electric fields. This could lead to development of new optoelectronic functions, more efficient logic processing units, more precise bond-selective control, new catalytic paradigms, and new chemical reactions.

Low Reducing Potentials Enabled by CaF2-Supported Graphene Electrodes in High Impedance Solutions

We report electrochemical measurements using in situ Raman spectroscopy at graphene/D2O interfaces under extremely low applied potentials. Here, the hydrophobic and catalytically inert nature of graphene and the insulating nature of the deionized (DI) water enables potentials as low as Vapplied = -7 V vs Ag/AgCl to be applied without exceeding 200 μA/cm2 of current density. At higher currents, bubble formation (i.e., hydrogen evolution reaction) prohibits reliable spectra from being obtained from the electrode surface. Using CaF2 as the supporting substrate enables significantly lower reducing potentials to be reached compared to glass substrates, likely due to trapped charge and impurities in the glass substrate. G band Raman spectra taken under various applied electrochemical potentials exhibit a linear relationship between the G band shift (ΔωG) and the applied potential, with blueshifts as high as ΔωG = 18 cm-1. These large Raman shifts indicate a large change in the Fermi level of ΔEF = -0.43 eV for graphene electrodes in contact with water, favoring reduction half-reactions. Based on the solution resistance measurement, there is a VIR = 3.1 V voltage drop across the solution for D2O (when the applied potential was Vapplied = -7 V vs Ag/AgCl) and the effective reducing potential on the working electrode is Veffective = -3.9 V vs Ag/AgCl. We have also tested these graphene electrodes in ionic liquids [DEME][TFSI], which are limited to applied potentials above Vapplied = -2.7 V vs Ag/AgCl and a corresponding shift in the Fermi level ΔEF = -0.32 eV, indicating that pure water can provide a more robust electrolyte for reaching low reducing potentials than ionic liquids.

Tailoring Interfaces for Enhanced Methanol Production from Photoelectrochemical CO2 Reduction

Efficient and stable photoelectrochemical reduction of CO2 into highly reduced liquid fuels remains a formidable challenge, which requires an innovative semiconductor/catalyst interface to tackle. In this study, we introduce a strategy involving the fabrication of a silicon micropillar array structure coated with a superhydrophobic fluorinated carbon layer for the photoelectrochemical conversion of CO2 into methanol. The pillars increase the electrode surface area, improve catalyst loading and adhesion without compromising light absorption, and help confine gaseous intermediates near the catalyst surface. The superhydrophobic coating passivates parasitic side reactions and further enhances local accumulation of reaction intermediates. Upon one-electron reduction of the molecular catalyst, the semiconductor-catalyst interface changes from adaptive to buried junctions, providing a sufficient thermodynamic driving force for CO2 reduction. These structures together create a unique microenvironment for effective reduction of CO2 to methanol, leading to a remarkable Faradaic efficiency reaching 20% together with a partial current density of 3.4 mA cm-2, surpassing the previous record based on planar silicon photoelectrodes by a notable factor of 17. This work demonstrates a new pathway for enhancing photoelectrocatalytic CO2 reduction through meticulous interface and microenvironment tailoring and sets a benchmark for both Faradaic efficiency and current density in solar liquid fuel production.

Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy for Probing Riboflavin on Graphene

Graphene research and technology development requires to reveal adsorption processes and understand how the defects change the physicochemical properties of the graphene-based systems. In this study, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and graphene-enhanced Raman spectroscopy (GERS) coupled with density functional theory (DFT) modeling were applied for probing the structure of riboflavin adsorbed on single-layer graphene substrate grown on copper. Intense and detailed vibrational signatures of the adsorbed riboflavin were revealed by SHINERS method. Based on DFT modeling and detected downshift of prominent riboflavin band at 1349 cm-1 comparing with the solution Raman spectrum, π-stacking interaction between the adsorbate and graphene was confirmed. Different spectral patterns from graphene-riboflavin surface were revealed by SHINERS and GERS techniques. Contrary to GERS method, SHINERS spectra revealed not only ring stretching bands but also vibrational features associated with ribityl group of riboflavin and D-band of graphene. Based on DFT modeling it was suggested that activation of D-band took place due to riboflavin induced tilt and distortion of graphene plane. The ability to explore local perturbations by the SHINERS method was highlighted. We demonstrated that SHINERS spectroscopy has a great potential to probe adsorbed molecules at graphene.